In the manufacture of urea fertilizer, from the top of the urea prilling tower of spray type or jet type is discharged an enormous quantity of air used for cooling of sprayed or jetted urea, as waste gas. The waste gas contains about 0.1 g/N m.sup.3 (waste gas) of fumes and about 200 ppm of NH.sub.3 gas and is at a temperature of 70.degree. C or thereabout.
Such dust content and toxic gas content are lower than the legal regulation limits, and also even when such a dust or a gas is dispersed in the atmosphere, they may not be considered as harmful because they are soluble in water. Further, the dust content as low as 0.1 g/N m.sup.3 is comparable to that obtained in the case where the highgrade treatment for dust removal is carried out by means of a dust collector such as an electrostatic precipitator, so that it can be hardly seen as smoke by the naked eye. But as in the case of the waste gas discharged from a urea prilling tower the dust is in the form of fumes whose particulate surface area is extremely large, the light seen through is scattered, and moreover, because of its quantity discharged being as extremely large as several hundred thousands N m.sup.3 /hr, when the waste gas is watched, a large number of fumes overlap each other causing the light to be scattered, whereby the fumes look like purply white or black smoke by the naked eye. Further, because of its temperature being as low as about 70.degree. C, the waste gas readily attains the atmospheric temperature after having been discharged, and thus the waste gas is characterized by that it can hardly diffuse, trailing over without disappearing for a long period of time after having been discharged.
For the above described reasons, in order to avoid the ill feeling of the inhabitants in the vicinity of the manufacturing factory of urea fertilizer it was necessary to remove the fumes in the waste gas to the extent that no fumes could be perceptible to the naked eye.
As a result of an experiment which was carried out by installing an experimental apparatus on the top of a urea prilling tower for dust collection it was found that in order to make the fumes in the waste gas indiscernible to the naked eye, the dust content should be reduced at least to less than 0.02 g/N m.sup.3, requiring more than 80% of efficiency in the dust collection.
Heretofore, in order to achieve the dust removal to such an extent by the use of a dust collector, use was made of a filtration dust collector such as a bag filter, but such a filter is undesirable in view of the fact that the pressure loss is so large that an enormous filtration area is required for a huge quantity of waste gas as much as several hundred thousands N m.sup.3 /hr to be treated, and there are various problems such as concerning the automatic regulation for the variation in the filtration resistance due to the pore clogging, the exchange of the filter cloth, etc. On the other hand, an electrostatic precipitator, which can perform the dust collection by the action of static electricity, is advantageous in that the pressure loss is small as well as the electric power consumption is small, but it is disadvantageous in that such precipitator requires a large volume and a large weight and also its cost is high, and so on. All these dry dust collectors are not only heavy in the weight but also high in the cost, so that none of them can satisfy the conditions that a huge quantity of waste gas containing fumes in a very low concentration (ca. 0.1 g/N m.sup.3) is treated in more than 80% of efficiency of dust collection by means of a dust collector, whose volume and weight of equipment are small and cost is low and moreover, which is capable of stable operation at low operation cost. Furthermore, they are disadvantageous in that some ammonia gas cannot be removed, and so on.
To remove at the same time a minute quantity of fumes and some quantity of ammonia gas contained in a huge quantity of waste gas there is also a method of dust collection by scrubbing (wet dust collection). This method of wet dust collection is far much more advantageous than the above described dry dust collection because the operation is not only very simple, lacking movable portions, but also very easy in cleaning, inspection, and repair, and the waste solution after scrubbing can also be circulated for reuse in such a way that if these water-soluble fumes and gas enter into solution and attain a certain concentration it is returned to the urea manufacturing plant without the necessity of disposing of the waste solution.
As the scrubbing dust collector which can remove fumes in a high efficiency there are known, heretofore, a venturi scrubber, a low pressure multistage venturi scrubber, a jet scrubber, etc., but those which can remove fumes in a high efficiency had usually any one of the deficiencies that the pressure loss is large, the liquid to gas ratio is large, or a high pressure pump is required for spraying the scrubbing solution.